In telecommunications networks, a well established way of handling calls is to use a CS connection between the telecommunications network and a user equipment (UE) for terminating a call. Further, there is the possibility of using a PS connection between the telecommunications network and a UE for terminating a call. For supporting the termination via the CS connection and handling CS calls, the telecommunications network is typically equipped with corresponding CS infrastructure, e.g., a switching node. Similarly, PS infrastructure may be provided for supporting the termination via the PS connection and handling packet based calls using Internet Protocol (IP) packets for transmitting media components and control signaling of a call. The PS infrastructure may for example be implemented by a packet telephony system (PTS) including one or more application servers. Some telecommunications networks may also provide both a CS telephony domain for handling CS calls and a PS telephony domain for handling PS calls.
An example of access technology which may be used for establishing a PS connection to a UE is the LTE (Long Term Evolution) radio access technology specified by the 3rd Generation Partnership Project (3GPP). Since this radio access technology does not support CS connections to the UE, a PTS may be used to provide telephony services using PS technology. In particular, a technologies referred to as IMS (IP Multimedia Subsystem) and VoLTE (Voice over LTE) may be used for implementing the PTS. In some cases, the IMS may also provide services to UEs which are connected using 2nd Generation (2G) radio access technology, e.g., GSM (Global System for Mobile Communications) radio access technology in connection with General Packet Radio Service (GPRS), or 3rd Generation (3G) radio access technology, e.g., UMTS (Universal Mobile Telecommunication System) radio access technology.
For unifying service experience, a concept referred to as IMS Centralized Services (ICS) was introduced. Details of the ICS concept can for example be found in 3GPP Technical Specification (TS) 23.292 V12.0 (2012-12). In this case telephony services for a UE are controlled by IMS in the same way as for VoLTE, even if the UE is connected via a 2G/3G access and attached to a switching node of the CS telephony domain referred to as Mobile Switching Center (MSC).
In one implementation of the ICS concept as specified in 3GPP TS 23.292, the UE is not specifically enhanced for ICS and attached to an MSC which is not specifically enhanced for ICS. In this case, all calls to or from the UE will be routed through the IMS, using the CS connection via the MSC to the UE. In such cases, the UE and the MSC will operate in the same way as for conventional CS calls which are routed through the CS telephony domain.
However, in such implementation problems may arise due to simultaneous execution of certain services both by the IMS and by the MSC. For example, a service referred to as Conditional Forwarding No Reply (CFNR) may be executed both by the IMS and by the MSC. A corresponding scenario is illustrated in FIG. 1, which involves IMS 11, MSC 12, and UE 13.
The scenario of FIG. 1 assumes that an incoming call from user A is directed to user B which is attached to MSC 12 and uses ICS implemented by IMS 11. User B is further assumed to have configured a CFNR service which forwards the calls to user C, making use of UE 13 if the call is not answered. This CFNR configuration is applied by IMS 11 and also by MSC 12.
As can be seen, due to the ICS implementation, the incoming call from user A first arrives at IMS 11, as indicated by Session Initiation Protocol (SIP) Invite message 21. IMS 11 then determines that user B is attached via a CS connection to MSC 12 and routes the call to MSC 12. This is accomplished by sending Integrated Services Digital Network User Part (ISUP) Initial Address Message (IAM) 22 toward MSC 12.
Upon receiving message 22, MSC 12 handles the call in the same way as a conventional CS call. This also involves starting a timer and, if the call is not answered by user B before the timer expires, forwarding the call to user C, i.e., to UE 13. As indicated by steps 23 and 24, the illustrated scenario assumes that the call is not answered by user B and is forwarded by MSC 12 to user C. For this purpose, MSC 12 proceeds by setting up a CS connection with UE 13, by sending a call setup command 25 toward the UE 13. As indicated by step 26, the call may then be answered at UE 13.
Due to the ICS configuration, the CFNR service is executed also at IMS 11. That is to say, also IMS 11 starts a timer and, if the call is not answered by user B before the timer expires, forwards the call to user C, i.e., to UE 13, as indicated by steps 27 and 28. The forwarding is in this case accomplished by first sending ISUP Release message (REL) 29 to MSC 12 to indicate that the termination toward user B should be released. Upon receiving ISUP REL 29, MSC 12 also clears the connection to user C, by sending a call release command 30 toward UE 13. User C will therefore not be able to receive the call, but rather encounter “ghost ringing” with no one on the other end, as indicated by step 31.
The call forwarding by IMS 11 also involves sending a call setup command 32 toward UE 13, to setup the call as forwarded to user C. However, since UE 13 is busy due to the ghost ringing, it returns busy indication 33 to the IMS 11. As indicated by step 34, IMS 11 will therefore conclude that the call cannot be setup due to UE 13 being busy. It is to be understood that also the call forwarding by the IMS 11 may involve interaction via the MSC 12. However, for the sake of clarity such interaction with the MSC 12 is not explicitly illustrated in FIG. 1.
Accordingly, due to the concurrent control of the CFNR service by the IMS 11 and the MSC 12, user A may encounter the line being busy, and user C may experience ghost ringing, which is an undesirable behavior.
Accordingly, there is a need for techniques which allow for efficiently handling calls in a telecommunications network having a CS telephony domain and a PS telephony domain.